Ion fluxes and ATPase activity are studied in intact cells,resealed ghosts and cell-free membranes, and 31P NMR spectroscopyis used to follow energetics in whole cells. Effects oftemperature on these measurements are studied in red cells of coldsensitive (human) and insensitive (ground squirrel) species. Wehave demonstrated that species differences in the effects oftemperature on free intracellular magnesium can account for muchif not all of the difference in temperature sensitivity of theNa/K pump. Studies using 19F NMR and a fluorinated calciumindicator have now begun in order to measure changes inintracellular free calcium as a function of